1. Field of the Invention
The present invention relates to a method of charging a secondary battery and an apparatus therefor and, more particularly, to a method of charging a lithium ion battery or similar non-aqueous secondary battery and an apparatus for the same.
2. Description of the Background Art
Today, small size, light weight and handy apparatuses implemented by advances in electronic technologies are widely used. Such handy apparatuses are powered by rechargeable secondary batteries. Handy phones and video cameras, for example, are loaded with lithium batteries or similar secondary batteries which are small size, light weight, and have a large capacity.
Japanese patent laid-open publication No. 123771/1992 (Document 1 hereinafter), for example, discloses the following method of charging a secondary battery. The method consists of continuously charging a secondary battery with a constant or semiconstant current until the battery reaches a preselected voltage, and thereafter repeatedly turning on and off a switch included in a charging path connected to the battery so as to repeat pulse charging. This method is directed toward a short charging time.
On the other hand, Japanese patent laid-open publication No. 274404/1995 (Document 2 hereinafter) teaches a charging apparatus including a circuit for determining that a protection circuit included in a battery pack has operated for protection. After the operation of the protection circuit, the apparatus sets up a charge voltage lower than a protection voltage assigned to the protection circuit or varies the charge voltage level periodically so as to repeat charging by cancelling the protective operation periodically.
The charging method taught in Document 1 leaves the following problems unsolved. The switch included in the charging path for effecting pulse charging after constant current charging is continuously turned on during constant current charging. As a result, a heavy thermal load acts upon a semiconductor device constituting the switch, resulting in a bulky charging apparatus. Moreover, the semiconductor device is periodically turned off during pulse charging in order to fully shut off the charge current. This degrades the charging efficiency for a unit time.
A problem with Document 2 is that a voltage cannot be sensed without resorting to voltage sense terminals provided on both of the battery pack and charging apparatus. This, coupled with the fact that the battery pack must switch its charge sense current and discharge sense current at the time of current sensing, makes the charging apparatus complicated in construction. Another problem is that the safety of the battery pack is degraded when, e.g., the circuit for detecting the operation of the protection circuit fails. In addition, the failure of such a circuit deteriorates the cycling efficiency of the battery.
As for an apparatus for charging a lithium ion secondary battery, constant current, constant voltage charging is effected while setting a limit on the charge current. In this case, the limit may be increased for a rapid charging purpose. However, this kind of scheme is not feasible without increasing the current rating of a switching device included in the charging apparatus, also increasing the size of the apparatus. Moreover, the charging time in the constant voltage charging range increases. In addition, the increase in charge current requires the protection circuit of the battery pack to be rearranged accordingly and brings about a temperature rise in the apparatus. This is undesirable from a safety standpoint.